The invention relates to a fertilizer and to a process for production thereof using as a starting material an organic waste such as, for example, sewage sludge or other organic household, industrial or agricultural waste. The process of the invention produces an organic fertilizer which is stable, does not harm the environment and is nitrogen enriched compared to the original waste. Further, the microorganism count in the final product is well below the safe upper limit set by the various regulatory authorities for such materials and is sufficiently low that the nitrogen content of the fertilizer is not reduced by microbial metabolism prior to application to the soil. The fertilizer of the invention has been shown to be as effective in improving crop yields as conventional inorganic fertilizers.
For many years now inorganic chemical fertilizers have dominated the fertilizer market. It has been more recently recognised, however, that exclusive use of inorganic fertilizers is harmful to the soil's natural ability to microbiologically replenish plant nutrients. They do not contain organic materials to replace top-soil lost through erosion and therefore, on application readily leach into reservoirs, rivers and lakes causing pollution.
In view of these drawbacks the use of fertilizers based on organic material is becoming increasingly popular because they can enrich the soil without substantial harm to the environment. Organic waste materials such as sewage sludge and other organic household, industrial and agricultural wastes are good candidates for turning into organic fertilizers because they are available in huge quantities and disposal of these wastes is, in itself, an environmental problem.
A number of methods are known for converting organic waste materials into organic fertilizers. However, a viable commercial process for production has proved elusive. Ideally, the process should fix or enrich the nitrogen content of the organic waste, hydrolyse organic components to increase their suitability to be metabolized by soil microorganisms and plants and reduce the microbial population of the organic waste. Various approaches have been taken to try and achieve those goals. For example, sterilization or disinfection of organic waste has been achieved by exposing the waste to high temperatures (pasteurization, drying) using an external heat source. Such a treatment, however, does not achieve any nutrient enrichment. Alternatively, the waste may be composted but this takes weeks or months and results in a voluminous material with a high water content which is inconvenient for use as a fertilizer.
Processes for enriching the nutrient content of organic wastes and bringing about hydrolysis of the organic components are also known. For example, U.S. Pat. No. 5,125,951 describes the conversion of ammonia in sewage sludge to thermally stable compounds such as ammonium nitrate and di-ammonium phosphate by treatment with nitric acid or phosphoric acid respectively. Another known method resulting in hydrolysis of the organic components and some disinfection of the waste is to treat the organic waste material with acid to hydrolyse the organic components and to drive down the pH. Thereafter alkali, usually ammonia, is added to raise the pH to one which would be suitable for application to the soil. The addition of ammonia increases the nitrogen content and the aggressive pH changes reduce the microbial count. Procedures using, for example, phosphoric acid or sulphuric acid to acidify the waste are described in U.S. Pat. No. 4,743,287, EP-A-0428014 and WO 91/16280. A similar process in which the acidifying agent is nitrogen dioxide is described in GB-424260.
A common feature of all these prior art processes is that acidification of the waste is carried out first before the addition of alkali to restore the pH to approximately neutral. The present inventors have now made the unexpected discovery that reaction times can be reduced and process efficiency substantially increased if the pH of the raw waste is first raised by adding alkali, followed by acidification with nitrogen dioxide. These efficiency improvements make production of a fertilizer from waste materials commercially feasible, particularly because some wastes, for example, sewage sludge, are often treated with alkali for stabilization and disinfection before transportation and disposal. Further, such a process achieves the dual goals of nitrogen enrichment and disinfection while producing a stable and environmentally acceptable product which may have greater than 50% of the total nitrogen content in the form of nitrites and nitrates.